1. Field of the Invention
The present invention relates to a surface illumination apparatus in which a laser light source is used as a source for light emission, and a liquid crystal display provided with this surface illumination apparatus.
2. Description of the Background Art
A liquid crystal display displays an image by controlling the transmission quantity of a beam of light projected from its rear surface, using an electro-optical effect by the orientation of liquid-crystal molecules. This method is employed, and thus in general, a backlight unit is needed which is formed by a cold-cathode fluorescent tube and the like. In such a liquid crystal display, in recent years, its screen size has become increasingly large, and even a 50-inch size display for television has been put to practical use. At the same time, however, its power consumption increases as its size is larger, and thus, the necessity arises for the development of an art of lessening the power consumption. As described above, a liquid crystal display is not a self-light-emission display, and thus, the power consumed in the light source of a backlight unit is greater than the power consumed in a liquid-crystal display panel.
In order to reduce the power consumption of the backlight-unit light source to a low level, a light source capable of emitting light efficiently needs to be used, as well as the backlight-unit illumination light needs to pass efficiently through the liquid-crystal display panel. These are grave problems to be solved. As such a light source, instead of a conventionally-used cold-cathode fluorescent tube, a light-emitting diode has been considered and already used in practice. However, numerous light-emitting diodes are necessary for obtaining illumination light which has a large area and a high intensity. Hence, an adequate reduction in the power consumption cannot be realized, though the range of color reproduction is enlarged to enhance the picture quality.
Therefore, as an important task, an art has to be developed of not wasting the illumination light of a backlight unit and allowing this illumination light to efficiently pass through a liquid-crystal display panel. In a liquid crystal display, illumination light projected from a backlight unit passes through the polarizing plate of a liquid-crystal display panel. Thereby, only half the quantity of light radiated from a light source is effectively used in practice. Hence, if illumination light polarized in advance passes through the polarizing plate, the illumination light could be efficiently used.
On the basis of this approach, as a first prior art, a configuration is disclosed in which the formation of a luminous layer and a waveguide is devised so that a polarized beam which can pass through a polarizing plate of a liquid-crystal display panel which forms a part of a liquid crystal display can be securely taken out beforehand from a light-emitting diode element (e.g., refer to Japanese Patent Laid-Open No. 2006-134975 specification). In this method, from the light-emitting diode element, a polarized beam whose wave is strongly polarized in parallel with the luminous layer and in the direction perpendicular to the emission direction from this element can be taken out. Therefore, effectively-used light which passes through the polarizing plate of the liquid-crystal display panel can be used at an efficiency of approximately 1.92 times as high as that of the case where random light is used. This would help reduce the power consumption of the liquid crystal display.
Furthermore, as a second prior art, a method is presented for heightening the usage efficiency of a light source by allowing the polarization-axis angle of a totally polarized beam or a partially polarized beam emitted from a backlight unit to coincide with the transmission-axis angle of a polarizing plate on the backside of a liquid-crystal display panel (e.g., refer to Japanese Patent Laid-Open No. 2005-242177 specification). Specifically, in a liquid crystal display according to any prior art, interfacial reflection or refraction is used when a beam of light emitted from a cold-cathode fluorescent tube or the like is converted into a surface-type beam of light. Thereby, a beam of light emitted from a backlight unit becomes not a natural beam but a totally polarized beam or a partially polarized beam.
Consequently, the angular relation between the polarization-axis angle of the totally polarized beam or the polarization-axis angle of the partially polarized beam's polarization component and the transmission-axis angle of the polarizing plate on the backside of the liquid-crystal display panel makes a difference in the quantity by which a beam of light from the backlight unit passes through the rear polarizing plate. This greatly affects the usage efficiency of a beam of light emitted from the backlight unit. On the other hand, in the illumination light, if the difference between the polarization-axis angle of the totally polarized beam or the polarization-axis angle of the partially polarized beam's polarization component and the transmission-axis angle of the polarizing plate is set within ten degrees, then the light source's usage efficiency could be enhanced.
Moreover, as a third prior art, a surface illumination apparatus is given which includes: a reflection mirror disposed opposite to a substrate; a point light source disposed sideward from this mirror; and an optical system that allows a beam of light from this point light source to turn into a parallel beam, go straight in parallel with the substrate and be incident on the reflection mirror, in which the parallel beam is reflected by the reflection mirror and is incident perpendicularly to a liquid-crystal cell portion (e.g., refer to Japanese Patent Laid-Open No. 10-104617 specification). According to this configuration, using the optical system, a beam of light emitted from the point light source is turned into a parallel beam. Hence, this parallel beam has an oblique light component less than any conventional one, thus helping heighten the usage efficiency of a beam of light and reduce the power consumption.
In addition, as a fourth prior art, another surface illumination apparatus is known which includes: a light source portion which emits illumination light with uniformly kept substantially-linearly polarized; and a light guide portion which allows the illumination light emitted from this light source portion to be incident on an incidence portion and emit this illumination light as surface light-source light from an emission portion, in which the incidence portion has a smaller area than the emission portion, and the light guide portion emits the illumination light incident on the incidence portion without changing its polarization state (e.g., refer to Japanese Patent Laid-Open No. 2006-202703 specification). According to this configuration, the usage efficiency of a beam of light becomes higher so that high-intensity illumination light can be emitted.
Furthermore, as a fifth prior art, a method is also disclosed for obtaining a high-definition and high-precision collimating plane light source, simply using a single laser beam (e.g., refer to Japanese Patent Laid-Open No. 2002-169480 specification). Specifically, it is configured by: a laser light source which has a predetermined emission width; a reflection member which reflects a beam emitted from this laser light source substantially in parallel in a predetermined direction; and a polarization member which includes a polarization plane that reflects the beam reflected by this reflection member in a substantially perpendicular direction to its parallel plane. According to this configuration, the collimating plane light source formed by only one laser beam can be obtained using a simple configuration. This makes it possible to obtain a liquid crystal display which has a high definition and a high efficiency.
However, in the above described first prior art, although the beam of a backlight unit is polarized in advance and is incident in the polarization direction of the polarizing plate, in this example, the light-emitting diode element is characterized by having a structure for strongly linearly-polarizing the beam. Hence, only a general configuration is described about a light guide plate.
In addition, in the second prior art, in the case where a natural beam is used, when it is reflected or refracted inside of a light guide plate, a beam of light emitted from the cold-cathode fluorescent tube or the like becomes a totally polarized beam or a partially polarized beam. Using this, the beam's usage efficiency can be improved. However, neither description nor suggestion is given about, in the case where an intrinsically polarized one such as a laser beam is used, emitting a parallel beam perpendicularly from a main surface with kept polarized.
Furthermore, in the third prior art, a beam of light emitted from the point light source is turned into a parallel beam by the optical system, is incident parallel to a liquid-crystal display panel and is incident perpendicularly to the liquid-crystal display panel by the reflection mirror. According to this configuration, however, because the reflection mirror is used, in the case where it is applied to a large-area liquid-crystal display panel, there is a disadvantage in that the surface illumination apparatus becomes thicker.
Moreover, in the fourth prior art, using the light source portion which emits linearly-polarized illumination light, the illumination light is incident on the end part of a light guide plate, and the illumination light at a high parallel level is emitted from a main surface. According to this configuration, however, it is characterized by including a group of several grooves on the backside of the light guide plate and emitting the illumination light in the light guide plate's normal direction by this groove group. However, in this example, the illumination light emitted from the light source portion is not a parallel beam, which requires optimizing the light guide plate's groove-group shape according to the optical-path length of the illumination light incident through its end part. Hence, a disadvantage arises in that the cost of producing the light guide plate increases.
In addition, in the fifth prior art, a parallel laser beam incident on the end part of a light guide plate partly passes through a half mirror provided inside of the light guide plate, and the rest is emitted substantially perpendicularly to the light guide plate's main surface. However, in order to emit illumination light having a uniform intensity from the main surface, a large number of half mirrors have to be arranged, as well as the transmittance of each such mirror needs to be gradually varied. This raises a disadvantage in that the production of the light guide plate including the half mirrors becomes complicated to increase its cost.
As described so far, in the prior arts, neither description nor suggestion is given about, using a laser light source, simplifying the configuration of a light guide plate, and simultaneously, preserving the polarization of a beam of light and allowing a parallel beam to irradiate a liquid-crystal display panel. Hence, they are still insufficient in performance and costs. Particularly, in recent years, in terms of a laser light source with a red beam (or R-beam), a green beam (or G-beam) and a blue beam (or B-beam), a high-power one has also been realized. This presents a desire for the development of a backlight unit which is capable of enhancing the usage efficiency of a beam of light using a laser light source.